Light receiving device with controllable sensitivity and spatial information detecting apparatus with charge discarding device using the same

ABSTRACT

A spatial information detecting apparatus using an intensity-modulated light has a photoelectric converter for receiving a light provided from a space, into which a light intensity-modulated by a predetermined modulation signal is being irradiated, and generating amounts of electric charges corresponding to an intensity of received light; charge discarding portion having an electrode for removing dispensable charges from the electric charges generated by the photoelectric converter according to a voltage applied to the electrode; charge storage portion for storing signal charges from the electric charges generated by the photoelectric converter; control circuit for controlling the voltage applied to the electrode at a timing synchronized with a period of the modulation signal to change a ratio of the signal charges stored in the charge storage portion to the electric charges generated by the photoelectric converter; charge ejector for outputting the signal charges from the charge storage portion; and an analyzer for determining spatial information from an output of the charge ejector. According to this apparatus, since the dispensable charges are previously removed from the electric charges generated by the photoelectric converter by the charge discarding portion, it is possible to improve S/N ratio and accurately determine the spatial information.

TECHNICAL FIELD

The present invention relates to a light receiving device using a newsensitivity control method and a spatial information detecting apparatusaccording to the technical concept of the same light receiving device.

BACKGROUND ART

In the past, a technique of detecting spatial information by use of anintensity-modulated light has been utilized. That is, theintensity-modulated light is irradiated from a light source into aspace, and a light reflected from an object in the space is received bya photoelectric converter. The spatial information can be determinedfrom a relationship between the intensity-modulated light and thereceived light. In the present description, the spatial informationincludes a distance from the object, a change in light receiving amountcaused by reflections on the object, and so on. For example, thedistance information can be determined from a phase difference betweenthe intensity-modulated light and the received light. In general, thistechnique is called as “Time-of-Flight (TOF)” method.

For example, U.S. Pat. No. 5,856,667 discloses a distance measuringapparatus using the TOF method. In this apparatus, a light emitted froma light source is intensity-modulated by a sine wave having a requiredmodulation frequency (i.e., emission frequency), and a light sensitivepart detects an intensity of received light plural times within a timeperiod shorter than a modulation period that is a reciprocal of themodulation frequency. When detecting the intensity of received light isrepeated 4 times within one modulation period, the phase difference isdetermined from the detected four intensities of received light. Forexample, when intensity-modulating the light irradiated from the lightsource to an object by a radio frequency wave of 20 MHz, a wavelength ofthe intensity-modulated light is 15 m. Therefore, when theintensity-modulated light goes to the object spaced from the apparatusby a distance of 7.5 m and back, a phase delay corresponding to onemodulation period occurs.

When the light emitted from the light source is intensity-modulated, asshown by the curve “W” of FIG. 30, and the modulated light reflectedfrom an object is received by the light sensitive part, the intensity ofreceived light changes, for example, as shown by the curve “R” of FIG.30. In this case, four intensities (A0, A1, A2, A3) of received lightcan be detected at 4 different phases (0°, 90°, 180°, 270°). However, inthe present circumstances, it is impossible to detect the intensity oflight received at just the moment of each of the phases (0°, 90°, 180°,270°). In the fact, each of the intensities of received lightcorresponds to the intensity of light received within a time width (Tw),as shown in FIG. 30.

On the assumption that the phase difference “ψ” does not change withinthe time period of sampling the intensity (A0, A1, A2, A3) of receivedlight, and there is no change in light extinction ratio between theemitted light and the received light, a relationship between theintensity (A0, A1, A2, A3) of received light and the phase difference“ψ” can be represented by the following equation:ψ=tan⁻¹{(A3−A1)/(A0−A2)}.

By use of the thus obtained phase difference “ψ” [rad], the modulationperiod “T” [s], and the speed of light “c” [m/s], a distance “L” [m]between the object and the apparatus can be calculated by the followingequation:L≈cT(ψ/4π).

To achieve the technical concept described above, this US patentproposes to use an image sensor shown in FIG. 31, which comprises fourmemory cells (M0, M1, M2, M3) provided every light sensitive part (PD),and an electrical switch (S0, S1, S2, S3) disposed between each of thememory cells and the light sensitive part. The electrical switches (S0,S1, S2, S3) are respectively turned on within the short time width (Tw),as shown in FIG. 30, to store the intensities (A0, A1, A2, A3) ofreceived light in the memory cells (M0, M1, M2, M3). By repeating thisprocedure with respect to a plurality of periods, it is possible toreduce influences of dark current noises, shot noises (i.e., noisescaused by variations in the occurrence of electron-hole pairs), staticnoises of an amplifier circuit, and so on, and improve S/N ratio. In thepresent specification, the operation described above is called as“synchronized integration”.

However, when electric charges are generated by the light sensitivepart, parts of them may remain in the light sensitive part for a momentwithout being transferred to the memory cell through the electricswitch. Such residual electric charges disappear by recombinations inthe light sensitive part. Alternatively, when another electrical switchis turned on within the time width (Tw), the residual electric chargesmay be accidentally transferred to another memory cell through to theelectric switch.

For example, when the modulation frequency is 20 MHz, it is needed thatthe time width (Tw) is shorter than the modulation period of 50 ns. Onthe other hand, time required for allowing the residual electric chargesto disappear by the recombinations is usually longer than 100 μs.Therefore there is a possibility that the residual electric charges areaccidentally transferred to another memory cell. This means that theresidual electric charges are mixed as noise components in signalcharges to be stored in the memory cell. As a result, when determiningthe phase difference “ψ” according to the above-described operation,there is a problem of lowering the accuracy of detecting the distanceinformation. In addition, when using the image sensor with a largenumber of electrical switches, as shown in FIG. 31, there is anotherproblem deteriorating cost/performance of the distance measuringapparatus.

SUMMARY OF THE INVENTION

Therefore, a primary concern of the present invention is to provide aspatial information detecting apparatus using an intensity-modulatedlight, which has the capability of preventing dispensable electriccharges being mixed as noise components in signal charges used todetermine the spatial information, and improving S/N ratio.

That is, the spatial information detecting apparatus of the presentinvention comprises:

-   at least one photoelectric converter for receiving a light provided    from a space, into which the light intensity-modulated by a    predetermined modulation signal is being irradiated, and generating    amounts of electric charges corresponding to an intensity of    received light;-   a charge discarding portion having a first electrode for removing    dispensable charges from the electric charges generated by the    photoelectric converter according to a voltage applied to the first    electrode;-   a charge storage portion for storing signal charges from the    electric charges generated by the photoelectric converter;-   a control circuit for controlling the voltage applied to the first    electrode at a timing synchronized with a period of the modulation    signal to change a ratio of the signal charges stored in the charge    storage portion to the electric charges generated by the    photoelectric converter;-   a charge ejector for outputting the signal charges from the charge    storage portion; and-   an analyzer for determining spatial information from an output of    the charge ejector.

According to the present invention, since the voltage applied to thefirst electrode of the charge discarding portion is controlled at thetiming-synchronized with the period of the modulation signal, there isan advantage that residual electric charges, which are not used as thesignal charges, i.e., parts of the electric charge generated by thephotoelectric converter can be surely removed as the dispensable chargesbefore they are transferred to the charge storage portion, and as aresult the S/N ratio is remarkably improved. In addition, since thesignal charges can be transferred the charge storage portion without thelapse of time needed for allowing the residual electric charges todisappear by recombinations in the photoelectric converter, it ispossible to efficiently determine the spatial information with highaccuracy.

In the present invention, it is preferred that the charge storageportion has a second electrode, and the control circuit controls avoltage applied to the second electrode constant to transfer requiredamounts of the electric charges generated by the photoelectric converterto the charge storage portion. Moreover, it is preferred that thecontrol circuit controls the voltages applied to the first and secondelectrodes so as to alternately switch between a stage of transferringthe electric charges generated by the photoelectric converter to thecharge storage portion and a stage of transferring the electric chargesgenerated by the photoelectric converter to the charge discardingportion.

In addition, it is preferred that the analyzer determines distanceinformation from the signal charges stored with respect to differentphases in a period of the modulation signal, and the spatial informationdetecting apparatus further comprises a phase switch for changing thephase of the modulation signal, at which the voltage is applied to thefirst electrode, every time that storing the signal charges in thecharge storage portion at the phase is finished. It is further preferredthat the charge storage portion has a light shielding film on the firstelectrode formed in the vicinity of a region of storing the signalcharges.

A further concern of the present invention is to provide another spatialinformation detecting apparatus using an intensity-modulated light,which has the capability of achieving the above-described advantages.

That is, the spatial information detecting apparatus of the presentinvention comprises:

-   at least one photoelectric converter for receiving a light provided    from a space, into which a light intensity-modulated by a    predetermined modulation signal is being irradiated, and generating    amounts of electric charges corresponding to an intensity of    received light;-   a charge discarding portion having a first electrode for removing    dispensable charges from the electric charges generated by the    photoelectric converter according to a voltage applied to the first    electrode;-   a charge storage portion having a second electrode for storing    signal charges from the electric charges generated by the    photoelectric converter according to a voltage applied to the second    electrode;-   a control circuit for controlling the voltage applied to the second    electrode at a timing synchronized with a period of the modulation    signal, while applying a constant voltage to the first electrode, to    change a ratio of the signal charges stored in the charge storage    portion to the electric charges generated by the photoelectric    converter;-   a charge ejector for outputting the signal charges from the charge    storage portion; and-   an analyzer for determining spatial information from an output of    the charge ejector.

According to the present invention, the ratio of the signal charges tobe stored in the charge storage portion to the electric chargesgenerated by the photoelectric converter can be changed by controllingthe voltage applied to the second electrode at the timing synchronizedwith the period of the modulation signal, and parts (i.e., residualelectric charges) of the electric charges generated by the photoelectricconverter are always discarded as the dispensable charges by applyingthe constant voltage to the first electrode. That is, it is possible tocontrol amounts of the signal charges to be transferred to the chargestorage portion from the electric charges generated by the photoelectricconverter, while removing the dispensable charges from the electriccharges generated by the photoelectric converter. Therefore, there is anadvantage of more efficiently determining the spatial informationwithout deteriorating the detection accuracy.

As a further preferred embodiment of the present invention, the spatialinformation detecting apparatus comprises a plurality of photoelectricconverters, and a set of photoelectric converters is selected from theplurality of photoelectric converters to define one pixel. The controlcircuit allows the charge storage portion to store the signal chargesfrom the electric charges generated by each of the photoelectricconverters of the set at a timing of each of different phases insynchronization with the period of the modulation signal. The chargeejector simultaneously outputs the signal charges stored with respect tothe different phases.

Another concern of the present invention is to provide a spatialinformation detecting method using a conventional CCD image sensorhaving an overflow drain electrode, which is characterized bycontrolling a control voltage applied to the overflow electrode insynchronization with the period of the modulation signal to achieve asubstantially same operation as the synchronized integration.

That is, the spatial information detecting method comprises the stepsof:

-   allowing the CCD image sensor to receive a light provided from a    space, into which a light intensity-modulated by a predetermined    modulation signal is being irradiated;-   storing signal charges by repeating a charge extraction operation    plural times with respect to each of different phases in a period of    the modulation signal; and-   determining spatial information from the signal charges stored with    respect to the different phases of the modulation signal,    -   wherein the charge extraction operation includes the steps of        removing dispensable charges from electric charges generated by        photoelectric converters of the CCD image sensor according to a        control voltage applied to the overflow electrode in        synchronization with the period of the modulation signal, and        storing the balance of the electric charges as the signal        charges in a charge storage area of the CCD image sensor.

According to the above method of the present invention, even when theconventional CCD image sensor is used to detect the spatial information,it is possible to improve S/N ratio and efficiently obtain the spatialinformation with high accuracy by controlling the control voltageapplied to the overflow electrode in synchronization with the period ofthe modulation signal in place of using a large number of electricalswitches.

In the detecting method described above, it is preferred that the CCDimage sensor has at least three photoelectric converters, and the chargeextraction operation includes the step of applying the control voltageto the overflow drain electrode in synchronization with the period ofthe modulation signal such that the electric charges generated by apredetermined one(s) of the at least three photoelectric converters arestored as the signal charges in the charge storage area, and theelectric charges generated by the remaining photoelectric converter(s)are discarded as the dispensable charges. It is also preferred that thecontrol voltage is applied to the overflow drain electrode to generate apotential barrier for electrically isolating the predeterminedphotoelectric converter(s) from the remaining photoelectricconverter(s).

Still another concern of the present invention is to provide a lightreceiving device with controllable sensitivity, which is particularlyuseful for the spatial information detecting apparatus described above.

That is, the light receiving device of the present invention comprises:

-   at least one photoelectric converter for receiving a light provided    from a space, into which a light intensity-modulated by a    predetermined modulation signal is being irradiated, and generating    amounts of electric charges corresponding to an intensity of    received light;-   a charge discarding portion having an electrode for removing    dispensable charges from the electric charges generated by the    photoelectric converter according to a voltage applied to the    electrode;-   a charge storage portion for storing signal charges from the    electric charges generated by the photoelectric converter;-   a sensitivity controller for controlling the voltage applied to the    electrode at a timing synchronized with a period of the modulation    signal to change a ratio of the signal charges stored in the charge    storage portion to the electric charges generated by the    photoelectric converter; and-   a charge ejector for outputting the signal charges from the charge    storage portion.

These and still other objects and advantages of the present inventionwill become more apparent from the best mode for carrying out theinvention explained below, referring to the attached drawings.

BRIEF EXPLANATION OF THE DRAWINGS

FIG. 1 is a block diagram of a distance measuring apparatus of thepresent invention;

FIGS. 2A to 2D are explanatory diagrams illustrating the principle ofoperation of the distance measuring apparatus according to a firstembodiment of the present invention;

FIGS. 3A to 3D are explanatory diagrams illustrating the principle ofoperation of the distance measuring apparatus according to a secondembodiment of the present invention;

FIGS. 4A to 4D are explanatory diagrams illustrating the principle ofoperation of the distance measuring apparatus according to a thirdembodiment of the present invention;

FIG. 5 is a plan view of an image sensor of a distance measuringapparatus according to a fourth embodiment of the invention;

FIG. 6 is an exploded perspective view of a relevant portion of theimage sensor.

FIG. 7 is a cross-sectional view cut along the line A—A of FIG. 6 or 11;

FIG. 8 is a potential diagram illustrating an operation of the distancemeasuring apparatus,

FIGS. 9A to 9C are potential diagrams illustrating another operations ofthe distance measuring apparatus;

FIG. 10 is a plan view of an image sensor of a distance measuringapparatus according to a fifth embodiment of the invention;

FIG. 11 is an exploded perspective view of a relevant portion of theimage sensor.

FIG. 12 is a potential diagram illustrating an operation of the distancemeasuring apparatus;

FIGS. 13A to 13C are potential diagrams illustrating another operationsof the distance measuring apparatus;

FIG. 14 is a plan view of an image sensor of a distance measuringapparatus according to a sixth embodiment of the invention;

FIG. 15 is a perspective view of a relevant portion of the image sensor.

FIG. 16 is a potential diagram illustrating an operation of the distancemeasuring apparatus;

FIG. 17 is potential diagram illustrating another operation of thedistance measuring apparatus;

FIGS. 18A to 18C are potential diagrams illustrating still anotheroperations of the distance measuring apparatus;

FIG. 19 is a plan view of an image sensor of a distance measuringapparatus according to a seventh embodiment;

FIG. 20 is a perspective view of a relevant portion of the image sensor.

FIG. 21 is a potential diagram illustrating an operation of the distancemeasuring apparatus;

FIGS. 22A to 22C are potential diagrams illustrating another operationsof the distance measuring apparatus;

FIGS. 23A and 23B are potential diagrams illustrating operations of adistance measuring apparatus according to an eighth embodiment of theinvention;

FIGS. 24A and 24B are potential diagrams illustrating operations of adistance measuring apparatus according to a ninth embodiment of theinvention;

FIGS. 25A and 25B are explanatory diagrams 3-dimensionally illustratingelectron potentials generated under gate electrodes within a chargestoring stage and a charge discarding stage, respectively;

FIGS. 26A and 26B are potential diagrams illustrating operations of adistance measuring apparatus according to a tenth embodiment of theinvention;

FIG. 27 is a perspective view of a distance measuring apparatusaccording to an eleventh embodiment of the invention;

FIG. 28 is a plan view of an image sensor of a distance measuringapparatus according to a twelfth embodiment of the invention;

FIG. 29 is a plan view of an image sensor of the distance measuringapparatus according to a thirteenth embodiment of the invention;

FIG. 30 is an explanatory diagram illustrating the principle ofoperation of a conventional distance measuring apparatus; and

FIG. 31 is a plan view of an image sensor used in the conventionaldistance measuring apparatus.

BEST MODE FOR CARRYING OUT THE INVENTION

As an example of a spatial information detecting apparatus of thepresent invention, preferred embodiments of a distance measuringapparatus for determining distance information from a phase differencebetween an intensity-modulated light irradiated to an object and a lightreceived by photoelectric converter are explained in detail below.However, needless to say, the present invention is not limited to thoseembodiments. It should be interpreted that the technical idea of thepresent invention is available to any apparatus or devices using thephase difference.

<First Embodiment>

As shown in FIG. 1, the distance measuring apparatus of the presentembodiment has a light source 2 for emitting a light into a requiredspace. The light emitted from the light source 2 is intensity-modulatedat a predetermined modulation frequency by a control circuit 3. As thelight source 2, for example, it is possible to use an array of lightemitting diodes (LED) or a combination of semiconductor laser anddivergent lens. As an example, the control circuit 3 intensity-modulatesthe light emitted from the light source 2 by a sine wave of 20 MHz.

In addition, the distance measuring apparatus has a plurality ofphotoelectric converters 11 for receiving a light provided from thespace through an optical system such as lens 4. Each of thephotoelectric converters 11 generates amounts of electric chargescorresponding to an intensity of received light. For example, aphotodiode can be used as the photoelectric converter 11. As a structureof the photodiode, there are “p-n” junction structure, “pin” structureand “MIS” structure. In this embodiment, a matrix array of 100×100photoelectric converters 11 is used as an image sensor 1.

3-dimensional information of the space, into which the light of thelight source 2 is being irradiated, is mapped on a 2-dimensional planarsurface that is a light receiving surface of the image sensor 1 throughthe lens 4. For example, when an object “Ob” exists in the space, alight reflected from each point of the object is received by aphotoelectric converter. By detecting a phase difference between thelight emitted from the light source 2 and the light received by thephotoelectric converter 11, a distance between the apparatus and eachpoint of the object can be determined.

In addition to the photoelectric converter 11, the image sensor 1 has acharge storage portion 12 for storing parts of electric chargesgenerated by the photoelectric converter 11 as signal charges used todetermine the phase difference, charge discarding portion 14 forremoving dispensable charges that are not used as the signal chargesfrom the electric charges generated by the photoelectric converter, anda charge ejector 13 for outputting the signal charges from the chargestorage portion 12 to the outside of the image sensor 1. The chargestorage portion 12 has a first electrode 12 a. Amounts of electriccharges transferred from the photoelectric converter 11 to the chargestorage portion 12 can be changed by controlling a voltage applied tothe first electrode 12 a. In addition, the charge discarding portion 14has a second electrode 14 a. Amounts of electric charges transferredfrom the photoelectric converter 11 to the charge discarding portion 14can be changed by controlling a voltage applied to the second electrode14 a.

The charge storage portion 12 is provided every photoelectric converter11. Therefore, the number of the charge storage charge portions 12 isequal to the number of the photoelectric converters 11. On the otherhand, the charge discarding portion 14 may be provided every set of aplurality of photoelectric converters 11. In this embodiment, a singlecharge discarding portion 14 is formed for all of the photoelectricconverters 11 of the image sensor 1. The signal charges output from thecharge ejector 13 are sent to an analyzer 5. In the analyzer 5, thephase difference between the light irradiated from the light source 2and the light received by the photoelectric converter 11 is determined,and a distance between the apparatus and the object can be determinedaccording to the phase difference.

As explained above, to determine the distance from the object “Ob”, itis required to detect the intensity (A0, A1, A2, A3) of received lightat the timing synchronized with a period of the modulation signal.Electric charges generated by the photoelectric converter 11 within aconstant time width “Tw” (FIG. 30) at a specific phase (e.g., 0°, 90°,180°, 270°) of the modulation signal are stored as the signal charges inthe charge storage portion 12. Therefore, it is preferred to increaseamounts of electric charges generated within the time width “Tw”, and onthe contrary decrease the amounts of electric charges generated in theremaining time periods other than the time width (Tw), and ideallyreduce the amounts of electric charges generated in the remaining timeperiods to zero. Since the amounts of electric charges generated by thephotoelectric converter 11 changes in accordance with an amount of lightincident on the photoelectric converter 11, it is required to controlsensitivity of the image sensor 1 to obtain increased amounts of signalcharges.

To control the sensitivity of the image sensor 1, it can be proposed tocontrol a magnitude of the voltage applied to the first electrode 12 aof the charge storage portion 12 at an adequate timing. However, sinceparts of the electric charges generated by the photoelectric converter11, i.e., residual electric charges that are not used as the signalcharges, may be mixed as noise components in the signal charges, it isnot sufficient to control the magnitude of the voltage applied to thefirst electrode.

In the present embodiment, required amounts of electric chargesgenerated by each of the photoelectric converters 11 are always suppliedto the corresponding charge storage portion 12 by keeping the magnitudeof the voltage applied to the first electrode 12 a constant. On theother hand, a required voltage is applied to the second electrode 14 aof the charge discarding portion 14 except for a signal chargegeneration period, in which the electric charges used as the signalcharges are generated by the photoelectric converter 11, so that theelectric charges generated in the time period other than the signalcharge generation period by the photoelectric converter 11 are sent asdispensable charges to the charge discarding portion 14. In short, thesensitivity of the image sensor 1 can be controlled by changing thevoltage applied to the second electrode 14 a at the timing synchronizedwith the period of the modulation signal.

The basic concept of the present invention is explained in more detailreferring to FIGS. 2A to 2D. In this explanation, the light irradiatedfrom the light source 2 to the space is intensity-modulated by amodulation signal shown in FIG. 2A. For example, electric chargescorresponding to an intensity (e.g., “A0” of FIG. 30) of received lightare generated by the photoelectric converter 11 in a specific phaserange (0 to 90 degrees) corresponding to the time width “Tw” of FIG. 30within one period of the modulation signal, and stored in the chargestorage portion 12. This procedure is repeated plural times, e.g.,several ten thousands of times˜several hundreds of thousand of times(i.e., several ten thousands of periods˜several hundreds of thousand ofperiods of the modulation signal) to accumulate signal chargescorresponding to the intensity “A0” of received light.

In this case, as shown in FIG. 2B, the voltage applied to the firstelectrode 12 a of the charge storage portion 12 is maintained constant.On the other hand, as shown in FIG. 2C, a required voltage is applied tothe second electrode 14 a of the charge discarding portion 14 for thetime period (corresponding to the phase range of 90 to 360 degrees)other than the signal charge generation period, so that the electriccharges generated by the photoelectric converter during the time periodare discarded as dispensable charges. In other words, since the voltageis not applied to the second electrode 14 a for the signal chargegeneration period (corresponding to the phase range of 0 to 90 degrees),the electric charges generated by the photoelectric converter 11 duringthe signal charge generation period is supplied as the signal charges tothe charge storage portion 12.

In this control method, therefore, the stage (FIG. 2B) of applying thevoltage to the first electrode 12 a of the charge storage portion 12 ispartially overlapped with the stage (FIG. 2C) of applying the voltage tothe second electrode 14 a of the charge discarding portion 14. Accordingto this voltage control, it is possible to extract the signal chargescorresponding to the intensity “A0” of received light, as shown in FIG.2D. In this embodiment, data sampling is performed every quarter ofperiod of the modulation signal.

The above signal extracting treatment shown in FIGS. 2A to 2D arerepeated several ten thousands of times˜several hundreds of thousand oftimes to obtain accumulated signal charges corresponding to theintensity “A0” of received light, and then the accumulated signalcharges are output to the analyzer 5 through the charge ejector 13.Similarly, the above procedures are performed to store the accumulatedsignal charges corresponding to each of the intensities (A0, A1, A2, A3)of received light in the charge storage portion 12.

Thus, since the image sensor 1 of the present embodiment has the chargediscarding portion 14 with the second electrode 14 a, and parts of theelectric charges generated by the photoelectric converter 11 that arenot used as the signal charges are aggressively removed as thedispensable charges by controlling the voltage applied to the secondelectrode 14 a, it is possible to effectively prevent the dispensablecharges being mixed as noise components into the signal charges. Inaddition, since the signal charges are accumulated over several tenthousands of periods to several hundred of thousands of periods of themodulation signal, it is possible to precisely determine the distanceinformation. From the viewpoint of high sensitivity, for example, whenthe modulation signal is set to 20 MHz, and the signal charges areextracted at 30 frames per second, it is possible to accumulate thesignal charges over several hundred of thousands of periods of themodulation signal.

In the above explanation, the constant voltage is applied to the firstelectrode 12 a during the time period of applying the voltage to thesecond electrode 14 a. However, by adequately setting a magnituderelation between the voltages applied to the first and secondelectrodes, it is possible to prevent the signal charges being storedduring the stage of removing the electric charges as the dispensablecharges.

As a modification of this embodiment, a set of the charge storageportion 12 and the charge discarding portion 14 may be formed everyphotoelectric converter 11. In this case, it is possible to obtain theelectric charges corresponding to four intensities (A0, A1, A2, A3) ofreceived light at a time within one period of the modulation signal. Asto the timing of data sampling, when phase intervals are predetermined,it is not required to adopt equally spaced phase intervals.

In the present embodiment, the light emitted from the light source 2 isintensity-modulated by the sine wave. However, the intensity modulationmay be performed by use of another waveform such as a triangular wave ora saw-tooth wave. In addition, as the light emitted from the lightsource 2, infrared light or the like may be used other than the visiblelight. In place of the image sensor 1 having the matrix arrangement ofthe photoelectric converters 11, another image sensor having a onedimensional arrangement of the photoelectric converters 11 may be used.In addition, when measuring the distance information in only onedirection in the space, or scanning the light beam irradiated from thelight source 2 to the space, it is possible to adopt the image sensorusing only four photoelectric converters to determine the distanceinformation. In this embodiment, the photoelectric converters 11 areintegrally formed with the charge storage portions 12. However,functions of the image sensor 1 may be achieved by use of separatedparts.

<Second Embodiment>

This embodiment presents another sensitivity controlling method for theimage sensor of the distance measuring apparatus shown in FIG. 1. Thatis, as shown in FIGS. 3A to 3D, the control method is characterized inthat a stage (FIG. 3B) of applying a voltage to the first electrode 12 aof the charge storage portion 12 is not overlapped with the stage (FIG.3C) of applying a voltage to the second electrode 14 a of the chargediscarding portion 14.

For example, the light irradiated from the light source 2 to the spaceis intensity-modulated by a modulation signal shown in FIG. 3A. FIGS. 3Bto 3C explain the case of detecting electric charges corresponding tothe intensity (e.g., “A0” of FIG. 30) of received light, which aregenerated by the photoelectric converter in a specific phase range (0 to90 degrees) corresponding to the time width “Tw” of FIG. 30 within oneperiod of the modulation signal. As shown in FIG. 3B, the voltage isapplied to the first electrode 12 a every quarter of period of themodulation signal, i.e., in a specific phase range (0 to 90 degrees)within each period of the modulation signal to send the electric chargesgenerated by the photoelectric converter 11 as the signal charges to thecharge storage portion 12.

On the other hand, as shown in FIG. 3C, the voltage is applied to thesecond electrode 14 a in the remaining phase range (90 to 360 degrees)other than the specific phase range (0 to 90 degrees), in which thevoltage is applied to the first electrode 12 a, to send the electriccharges generated by the photoelectric converter 11 as the dispensablecharges to the charge discarding portion 14. According to the voltagecontrol described above, it is possible to extract the signal chargescorresponding to the intensity “A0” of received light, as shown in FIG.3D.

According to this embodiment, it is possible to separately control thevoltages applied to the first and second electrodes without consideringthe magnitude relation between those voltages, and therefore there is anadvantage that it becomes easy to control these voltages. As a result,it is possible to easily control the sensitivity that is a ratio of thesignal charges relative to the electric charges generated by thephotoelectric converter 11, and the ratio of the dispensable chargesrelative to the electric charges generated by the photoelectricconverter 11.

In this embodiment, since the stage of storing the signal charges in thecharge storage portion 12 is determined by the voltage applied to thefirst electrode 12 a, it is possible to shorten the stage of applyingthe voltage to the second electrode 14 a. For example, the voltage maybe applied to the second electrode 14 a for a required time periodimmediately before the voltage is applied to the first electrode 12 a.Other configuration and operation are substantially the same as thefirst embodiment.

<Third Embodiment>

This embodiment presents another sensitivity controlling method for theimage sensor 1 of the distance measuring apparatus shown in FIG. 1. Thatis, as shown in FIGS. 4A to 4D, the control method is characterized inthat a constant voltage is always applied to the second electrode 14 aof a charge discarding portion 14 to discard parts of the electriccharges generated by the photoelectric converter 11 as dispensablecharges, and the electric charges generated by the photoelectricconverter 11 are stored as signal charges in the charge storage portion12 in only a stage of applying a voltage to a first electrode 12 a ofthe charge storage portion 12.

For example, the light irradiated from the light source 2 to the spaceis intensity-modulated by a modulation signal shown in FIG. 4A. FIGS. 4Bto 4C explain the case of detecting electric charges corresponding tothe intensity (e.g., “A0” of FIG. 30) of received light, which aregenerated by the photoelectric converter in a specific phase range (0 to90 degrees) corresponding to the time width “Tw” of FIG. 30 within oneperiod of the modulation signal. As shown in FIG. 4B, the voltage isapplied to the first electrode 12 a every quarter of period of themodulation signal, i.e., in a specific phase range (0 to 90 degrees)within each period of the modulation signal to send the electric chargesgenerated by the photoelectric converter 11 as the signal charges to thecharge storage portion 12.

On the other hand, as shown in FIG. 4C, a constant DC voltage is alwaysapplied to the second electrode 14 a to send parts of the electriccharges generated by the photoelectric converter 11 as the dispensablecharges to the charge discarding portion 14. However, as describedabove, since the electric charges generated by the photoelectricconverter 11 in the stage of applying the voltage to the first electrode12 a are sent as the signal charges to the charge storage portion 12, itis possible to extract the signal charges corresponding to the intensity“A0” of received light, as shown in FIG. 4D.

According to this embodiment, irrespective of the presence or absence ofthe voltage applied to the first electrode 12 a, predetermined amountsof the electric charges are always discarded to the charge discardingportion 14 by applying the constant voltage to the second electrode 14a. Therefore, the dispensable charges (residual electric charges) notused as the signal charges can be surely removed by the chargediscarding portion 14.

By the way, even when the electric charges generated by thephotoelectric converter 11 are stored as the signal charges in thecharge storage portion 12 by applying the voltage to the first electrode12 a, the predetermined amounts of electric charges are sent as thedispensable charges to the charge discarding portion 14. Therefore, inthe strict sense, to store sufficient amounts of the signal charges inthe charge storage portion 12, it is needed to consider the magnituderelation between the voltages applied to the first and secondelectrodes. However, in a practical sense, since the voltage applied tothe second electrode is constant, it is sufficient to control only thevoltage applied to the first electrode. Other configuration andoperation are substantially the same as the first embodiment.

<Fourth Embodiment>

This embodiment explains about a case of using an interline transfer CCDhaving a vertical-overflow drain, which is available in the market, asthe image sensor 5 of FIG. 1.

As shown in FIG. 5, the image sensor 1 is a 2-dimensional image sensorhaving a matrix arrangement of 3×4 photodiodes 21. The numeral 22designates a vertical transfer portion composed of a vertical transferCCD, which is provided adjacent to the photodiodes 21 of each column ofthe matrix arrangement. The numeral 23 designates a horizontal transferportion composed of a horizontal transfer CCD, which is provided at alower side of the vertical transfer portions. In each of the verticaltransfer portions 22, a pair of gate electrodes (22 a, 22 b) areprovided every photodiode 21. In the horizontal transfer portion, a pairof gate electrodes (23 a, 23 b) are provided every vertical transferportion 22.

The vertical transfer portion 22 is controlled by a 4-phase drive, andthe horizontal transfer portion 23 is controlled by a 2-phase drive.That is, four phase control voltages (V1 to V4) are applied to the gateelectrodes (22 a, 22 b) of the vertical transfer portion 22, and twophase control voltages (VH1, VH2) are applied to the gate electrodes (23a, 23 b) of the horizontal transfer portion 23. Since this type ofdriving technique is well known in the field of conventional CCD, afurther detail explanation is omitted.

The photodiodes 21, the vertical transfer portions 22 and the horizontaltransfer portion 23 are formed on a single substrate 20. An overflowelectrode 24 that is an aluminum electrode is directly formed on thesubstrate 20 not through an insulating film so as to surround the wholeof the photodiodes 21, the vertical transfer portions 22 and thehorizontal transfer portion 23, as shown in FIG. 5. When a requiredmagnitude of a positive voltage (Vs) is applied to the overflowelectrode 24, electric charges (electrons) generated by the photodiodes21 are discarded through the overflow electrode 24.

That is, in this embodiment, the substrate 20 is used as a part of theoverflow drain. Thus, since the overflow drain removes dispensablecharges from the electric charges generated by the photodiodes 21 of thephotoelectric converters 11, it functions as the charge discardingportion 14. Amounts of dispensable charges removed can be controlled bya voltage applied to the overflow electrode 24. Therefore, the overflowelectrode 24 functions as the second electrode 14 a.

Referring to FIG. 6, the image sensor 1 is more specifically explained.In the present embodiment, an n-type semiconductor substrate is used asthe substrate 20. A p-well 31 of p-type semiconductor is formed on ageneral surface of the substrate 20 over regions for the photodiode 21and the vertical transfer portion 22 such that a thickness of the p-wellat the region for the vertical transfer portion 22 is greater than thethickness of the p-well at the region for the photodiode 21. Inaddition, an n⁺-type semiconductor layer 32 is formed on the p-well 31at the region for the photodiode 21. As a result, the photodiode 21 iscomposed of a p-n junction formed by the n⁺type semiconductor layer 32and the p-well 31. A p⁺-type semiconductor surface layer 33 is formed onphotodiode 21. A purpose of forming the surface layer 33 is to preventthat when sending the electric charges generated by the photodiodes 21to the vertical transfer portion 22, a surface region of the n⁺-typesemiconductor layer 32 functions as a passage for the electric charges.This kind of photodiode is known as a buried photodiode.

On the other hand, a storage transfer layer 34 of n-type semiconductoris formed at the region corresponding to the vertical transfer portion22 on the p-well 31. The top surface of the storage transfer layer 34 issubstantially flush with the top surface of the surface layer 33, andthe thickness of the storage transfer layer 34 is greater than thethickness of the surface layer 33. A side of the storage transfer layer34 contacts the surface layer 33. A separation layer 35 of p⁺-typesemiconductor having the same impurity concentration as the surfacelayer 33 is formed between the n⁺-type semiconductor layer 32 and thestorage transfer layer 34. The gate electrodes 22 a, 22 b are formed onthe storage transfer layer 34 through an insulating film 25. The gateelectrode 22 a is insulated from the gate electrode 22 b by theinsulating film 25. As described above, the gate electrodes 22 a, 22 bare formed every photodiode 21. One of the gate electrodes 22 a, 22 bhas a larger width than the other one.

Specifically, as shown in FIG. 7, the gate electrode 22 b having thesmall width is configured in a planar shape, and the gate electrode 22 ahaving the large width is formed with a flat portion and a pair ofcurved portions extending from opposite ends of the flat portion. Thegate electrodes 22 a, 22 b are disposed such that the curved portion ofthe gate electrode 22 a is partially overlapped with the gate electrode22 b in the height direction. In addition, the top surface of the flatportion of the gate electrode 22 a is substantially flush with the topsurface of the gate electrode 22 b. Therefore, the gate electrodes 22 a,22 b are alternately disposed on the storage transfer layer 34 in thelength direction of the vertical transfer portion 22. The insulatingfilm 25 is made of silicon dioxide. The gate electrodes 22 a, 22 b aremade of polysilicon. In addition, a surface of the substrate 20 otherthan areas corresponding to the photodiodes 21 is covered with alight-shielding film 26.

Next, a mechanism of driving the image sensor 1 described above isexplained. When the light provided from the space is incident on thephotodiode 21, electric charges are generated by the photodiode 21. Inaddition, a ratio of the electric charges supplied as the signal chargesinto the vertical transfer portion 22 relative to the electric chargesgenerated by the photodiodes 21 can be determined according to arelationship between the voltage applied to the gate electrode 22 a andthe voltage applied to the overflow electrode 24. Specifically, theabove-described ratio is determined in accordance with a relationshipbetween a depth of a potential well formed in the storage transfer layer34 according to the voltage applied to the gate electrode 22 a and atime period of applying the voltage to the gate electrode 22 a, and apotential gradient formed between the photodiode 21 and the substrate 20according to the voltage applied to the overflow electrode 24 and a timeperiod of applying the voltage to the overflow electrode 24.

The voltages applied to the gate electrode 22 a (corresponding to thefirst electrode 12 a) and the overflow electrode 24 (corresponding tothe second electrode 14 a) can be controlled according to one of thevoltage control methods explained in the first to third embodiments. Inthis embodiment, those voltages are controlled by the method of thefirst embodiment, i.e., such that the stage of applying the voltage tothe gate electrode 22 a is partially overlapped with the stage ofapplying the voltage to the overflow electrode 24, as shown in FIGS. 2Ato 2D.

A ratio of the electric charges supplied to the vertical transferportion 22 relative to the electric charges generated by the photodiode21 changes in accordance with the voltages applied to the gateelectrodes 22 a, 22 b. That is, at the vertical transfer portion 22,since the gate electrodes 22 a, 22 b are formed on the storage transferlayer 34 through the insulating film 25, a potential well is formed inthe storage transfer portion 34 at the region corresponding to each ofthe gate electrodes 22 a, 22 b by applying the voltages to the gateelectrodes 22 a, 22 b. As a result, the electric charges can be storedin a capacity determined by depth and width of the potential well. Thus,the potential well functions as the charge storage portion 12 forstoring the signal charges.

In addition, the electric charges stored in the vertical transferportion 22 can be output to the analyzer 5 through the horizontaltransfer portion 23 by controlling a magnitude of the voltages appliedto the gate electrodes 22 a, 22 b, 23 a, 23 b and the timings ofapplying the voltages to the respective gate electrodes. Therefore, thevertical transfer portion 22 and the horizontal transfer portion 23function as the charge ejector 13.

For example, by controlling the voltage applied to the overflowelectrode 24, the electric charges generated by the photodiode 21 areallowed to migrate toward the vertical transfer portion 22, as describedbelow. That is, FIG. 8 is a schematic diagram showing a change inelectron potential along the dotted-line “L1” in FIG. 6. A regiondesignated by the numeral 21 in FIG. 8 corresponds to the photodiode. Aregion designated by the numeral 20 in FIG. 8 corresponds to thesubstrate. A region designated by the numeral 22 in FIG. 8 correspondsto the vertical transfer portion. When the voltage is not applied to theoverflow electrode 24, a potential barrier “B1” is formed by the p-well31 between the photodiode 21 and the substrate 20. In addition, when thevoltages are not applied to the gate electrodes 22 a, 22 b, a potentialbarrier “B2” is formed by the separation layer 35 between the photodiode21 and the vertical transfer portion 22. Therefore, the height of thepotential barrier “B2” can be changed by controlling magnitudes of thevoltages applied to the gate electrodes 22 a, 22 b, and the height ofthe potential barrier “B1” can be changed by controlling a magnitude ofthe voltage applied to the overflow electrode 24. In FIG. 8, “e”designates electron.

FIGS. 9A to 9C show relationships between voltages applied to the gateelectrode 22 a and the overflow electrode 24 and movements of theelectric charges generated by the photodiode 21. In FIG. 9A, thepotential barrier “B2” formed by the separation layer 35 is removed byapplying a relatively high positive voltage (V1) to the gate electrode22 a, and a potential well 27 is formed in the storage transfer layer34. At this time, a relatively low voltage (Vs) is applied to theoverflow electrode 24 to form the potential barrier “B1”. The electriccharges generated by the photodiode 21 can not migrate toward thesubstrate 20 by the presence of the potential barrier “B1”. As a result,the electric charges can not be discarded, and therefore the electriccharges generated by the photodiode 21 migrate as the signal charges tothe vertical transfer portion 22 as the capacity of the potential well27 allows.

On the other hand, in FIG. 9B, a relatively high positive voltage (V1)is applied to the gate electrode 22 a, as shown in FIG. 9A, and also arelatively high positive voltage (Vs) is applied to the overflowelectrode 24. The voltage applied to the overflow electrode 24 isdetermined such that a potential of the substrate 20 is lower than thepotential of the vertical transfer portion 22. Since the potentialbarrier “B1” formed by the p-well 31 is removed, and a potentialgradient of the substrate 20 against the photodiode 21 is greater thanthe potential gradient of the vertical transfer portion 22 against thephotodiode 21, most of the electric charges generated by the photodiodes21 migrate as the dispensable charges to the substrate 20, and thendiscarded, as shown by the arrow in FIG. 9B.

That is, the ratio of the signal charges in the electric chargesgenerated by the photodiode 21 remarkably reduces, as compared with thecase of FIG. 9A. This means a reduction in sensitivity of thephotoelectric converter 11. The sensitivity defined as a ratio of thesignal charges and the dispensable charges is determined by themagnitude relation between the voltages applied to the gate electrode 22a and the overflow electrode 24. Since the electric charges (electrons)easily migrate toward a lower electron potential, the electric chargessupplied into the vertical transfer portion 22 are stored in thepotential well 27 having a lower potential than the photodiode 21, asshown in FIG. 9A, and the stored electric charges can not migrate towardthe substrate 20, as shown in FIG. 9B.

To read out the signal charges stored in the vertical transfer portion22, the applied voltage (V1) is removed from the gate electrode 22 a (ora relatively small voltage (V1) may be applied to the gate electrode 22a) to form the potential barrier “B2”. In addition, a relatively smallvoltage (Vs) is applied to the overflow electrode 24 to form thepotential barrier “B1”. The formation of the potential barrier “B1” isnot necessarily required. It is important to form the potential barrier“B2”. The formation of the potential barrier “B2” inhibits an inflow ofthe electric charges from the photodiode 21 to the vertical transferportion 22, and an outflow of the electric charges from the verticaltransfer portion 22 to the photodiode 21. Under this condition, thesignal charges stored in the vertical transfer portion 22 are sent tothe analyzer 5 through the horizontal transfer portion 23.

The signal charges stored in the vertical transfer portion 22 are readout every time that the signal charges corresponding to one intensity(A0˜A3) of received light are detected. For example, after the signalcharges corresponding to the integral “A0” are stored in the potentialwell 27, they are read out. Next, after the signal charges correspondingto the integral “A1” are stored in the potential well 27, they are readout. Thus, the procedures of storing and reading out the signal chargesare repeated. It goes without saying that the time period of storing thesignal charges corresponding to the intensity (A0˜A3) of received lightis constant. In addition, the sequence of reading out the signal chargescorresponding to the intensity (A0˜A3) of received light is not limitedto the above-described case. For example, after the signal chargescorresponding to the intensity (A0) of received light is extracted, thesignal charges corresponding to the intensity (A2) of received light maybe extracted. Other configuration and operation are substantially thesame as the first embodiment.

<Fifth Embodiment>

This embodiment explains about a case of using an interline transfer CCDhaving a lateral-overflow drain, which is available in the market, asthe image sensor 1 of FIG. 1.

In the image sensor 1 of this embodiment, as shown in FIG. 10, anoverflow drain 41 of n-type semiconductor is formed adjacent to each ofcolumns of a matrix arrangement (3×4) of the photodiodes 21. In thiscase, the image sensor 1 has three overflow drains 41. The overflowdrains 41 are connected at their upper ends to each other by an overflowelectrode 24 that is an aluminum electrode. The vertical transferportion 22 and the horizontal transfer portion 23 have substantiallysame functions to them of the image sensor 1 of the fourth embodiment.

Referring to FIG. 11, the image sensor 1 is more specifically explained.In the present embodiment, a p-type semiconductor substrate is used asthe substrate 40. An n⁺-type semiconductor layer 42 is formed in ageneral surface of the substrate 40 over a region, at which theformation of the photodiode 21 is intended. Therefore, the photodiode 21is composed of the n⁺-type semiconductor layer 42 and the substrate 40.On the other hand, a storage transfer layer 44 of n-type semiconductoris formed in the general surface of the substrate 40 over a region, atwhich the formation of the vertical transfer portion 22 is intended.

A separation layer 45 a of p⁺-type semiconductor is formed between then⁺-type semiconductor layer 42 and the storage transfer layer 44. Aseparation layer 45 b of p⁺-type semiconductor is formed between then⁺-type semiconductor layer 42 and the overflow drain 41. The numeral 43designates a p⁺-type semiconductor surface layer having the sameimpurity concentration as the separation layers 45 a, 45 b, which isformed on the n⁺-type semiconductor layer 42 and the separation layers45 a, 45 b. This surface layer 43 prevents that the electric chargesgenerated by the photodiode 21 migrate to the vertical transfer portion22 through the surface of the n⁺-type semiconductor layer 42.

A top surface of the storage transfer layer 44 is substantially flushwith the top surfaces of the surface layer 43 and the overflow drain 41.In addition, a thickness of the overflow drain 41 is larger than thethickness of the n⁺-type semiconductor layer 42. Gate electrodes 22 a,22 b are formed every photodiode 21 on the storage transfer layer 44through an insulating film 25. One of the gate electrodes 22 a, 22 b hasa larger width than the other one. In addition, the surface of the imagesensor 1 other than areas for allowing the photodiodes 21 to receive thelight is covered with a light-shielding film 26. These are same as theimage sensor 1 of the fourth embodiment.

As understood from comparison between FIG. 12 showing a change inelectron potential along the dotted line “L2” in FIG. 11, FIGS. 13A to13C and

FIGS. 8, 9A to 9C, the present embodiment is different from the fourthembodiment only in that the electric charges generated by the photodiode21 are discarded by the overflow drain 41 in place of the substrate 20of the fourth embodiment. In addition, as compared with the n⁺-typesemiconductor layer 32 constructing the photodiode 21 of the interlinetransfer CCD having the vertical-overflow drain according to the fourthembodiment, the photodiode 21 of the interline transfer CCD having thelateral-overflow drain according to the present embodiment can be formedby the n⁺-type semiconductor layer 42 having a larger thickness.

That is, in the case of forming the vertical overflow drain, it isneeded to form the photodiode 21 on the substrate 20. However, in thecase of forming the lateral-overflow drain, since the substrate 40 isused as the semiconductor layer constructing the photodiode 21, itbecomes possible to increase the thickness of the n⁺-type semiconductorlayer 42. In addition, there is an advantage that the sensitivity toinfrared is improved, as compared with the fourth embodiment. Otherconfiguration and performance are substantially the same as the fourthembodiment.

<Sixth Embodiment>

This embodiment explains about a case of using a frame transfer CCDhaving a vertical-overflow drain, which is available in the market, asthe image sensor 1 of FIG. 1.

As shown in FIG. 14, the image sensor 1 of this embodiment is a2-dimensional image sensor having a matrix (4×4) arrangement ofphotodiodes 21. This image sensor 1 is provided with an image pickupportion “D1” including a plurality of vertical transfer CCDs, each ofwhich is composed of the photodiodes 21 aligned in the verticaldirection, and a storage portion “D2” formed adjacent to the imagepickup portion “D1”, which is an array of vertical transfer CCDs nothaving the photoelectric conversion capability. In FIG. 14, the numeral23 designates a horizontal transfer portion formed adjacent to thestorage portion “D2”, which is composed of by a horizontal transfer CCD.The horizontal transfer portion functions as a charge ejector.

In this embodiment, the photodiode 21 and the vertical transfer CCD havethe capability of storing electric charges and transferring the electriccharges in the vertical direction. Therefore, the image pickup portion“D1” and the storage portion “D2” function as a charge storage portion.Each of the columns of the image pickup portion “D1” has fourphotodiodes 21, each of which is formed at the light receiving surfacewith three gate electrodes 21 a˜21 c aligned in the vertical direction.Each of the columns of the storage portion “D2” has two sets of threegate electrodes 28 a˜28 c. In addition, in the horizontal transferportion 23, a pair of gate electrodes 23 a, 23 b are formed every columnof the storage portion “D2”, as in the case of the fourth embodiment.

The gate electrodes 21 a˜21 c are driven by 6-phase control voltages“V1”˜“V6”, and the gate electrodes 28 a˜28 c are driven by 3-phasecontrol voltages “VV1”˜“VV3”. The gate electrodes 23 a, 23 b are drivenby 2-phase control voltages “VH1”, “VH2”. The horizontal transferportion 23 outputs the signal charges from the storage portion “D2”every one horizontal line. Since this type of driving technique is wellknown in the field of CCDs, further detailed explanation is omitted.

The image pickup portion “D1”, the storage portion “D2” and thehorizontal transfer portion 23 are formed on a single substrate 50. Anoverflow electrode 24 that is an aluminum electrode is formed directlyon the substrate 50 not through an insulating film. That is, thesubstrate 50 functions as an overflow drain. The overflow electrode 24is formed on the substrate 50 so as to surround the whole of the imagepickup portion “D1”, the storage portion “D2” and the horizontaltransfer portion 23. A surface of the substrate 50 other than areascorresponding to the photodiodes 21 is covered with a light shieldingfilm (not shown).

Referring to FIG. 15, the image sensor 1 is more specifically explained.In the present embodiment, an n-type semiconductor substrate is used asthe substrate 50. A p-type semiconductor layer 51 is formed on a generalsurface of the substrate 50 over a region, at which the formation of thephotodiode 21 is intended. In addition, an n-well 52 of n-typesemiconductor is formed in the p-type semiconductor layer 51. The threegate electrodes 21 a˜21 c are formed on top surfaces of the p-typesemiconductor layer 51 and the n-well 52 through an insulating film 53of silicon dioxide. That is, in this embodiment, a MIS-type photodiode21 is formed by the n-well 52, the insulating film 53 and the gateelectrodes 21 a to 21 c. Each of the gate electrodes 21 a˜21 c is madeof polysilicon. The n-well 52 is continuously formed over the imagepickup portion “D1” and the storage portion “D2”. In the n-well 52 ofthe image pickup portion “D1”, the electric charges are generated,stored and transferred, and in the n-well 52 of the storage portion“D2”, the electric charges are stored and transferred.

Next, a mechanism of driving the image sensor 1 described above isexplained. When the light provided from the space is incident on thephotodiode 21, electric charges are generated by the photodiode 21. Whensuitable voltages are applied to the gate electrodes 21 a˜21 c, apotential well is formed as the charge storage portion in the n-well 52.Therefore, the generated electric charges can be stored in the potentialwell. In addition, the depth of the potential well can be changed bycontrolling the voltages applied to the gate electrodes 21 a˜21 c totransfer the electric charges. On the other hand, when a suitablevoltage “Vs” is applied to the overflow electrode 24, the electriccharges generated by the photodiode 21 are discarded through thesubstrate 50. Therefore, by controlling the voltage applied to theoverflow electrode 24 and a time period of applying the voltage to theoverflow electrode 24, it is possible to change a ratio of the signalcharges stored in the potential well of the n-well 52 relative to theelectric charges generated by the photodiode 21.

Referring to FIG. 16 showing a change in electron potential along thedotted line “L3” in FIG. 15, migrations of electric charges generated bythe photodiode 21 are explained. The region designated by the numeral 21in FIG. 16 corresponds to the photodiode. The region designated by thenumeral 50 in FIG. 16 corresponds to the substrate. When no voltage isapplied to the overflow electrode 24, a potential barrier “B3” is formedby the p-type semiconductor layer 51 between the photodiode 21 (n-well52) and the substrate 50. On the other hand, at the opposite side of thephotodiode 21, a potential barrier “B4” is formed by the p-typesemiconductor layer 51 to prevent a leakage of the electric charges(electrons “e”) generated by the photodiode 21 to the outside. A heightof the potential barrier “B3” can be changed by controlling a magnitudeof the voltage applied to the overflow electrode 24.

On the other hand, amounts of electric charges stored in the potentialwell, which is formed in the n-well 52 by applying the voltages to thegate electrodes 21 a˜21 c, are determined according to the depth of thepotential well, which can be changed by controlling magnitudes of thevoltages applied to the gate electrodes 21 a˜21 c. That is, when themagnitude of the voltage applied to the gate electrode 21 b is higherthan the magnitudes of the voltages applied to the gate electrodes 21 a,21 c, the potential well is formed to have a maximum depth at its centerregion, as shown in FIG. 17.

By applying a suitable voltage to the overflow electrode 24 such thatthe potential of the substrate 50 is lower than the potential of then-well 52, voltages to the gate electrodes 21 a, 21 c to remove thepotential barrier “B3”, and a voltage to the gate electrode 21 b to formthe potential barrier “B3”, as shown in FIGS. 18A to 18C, it is possibleto store large amounts of electric charges (electrons “e”) at the centerregion of the potential well shown in FIG. 18B, and discard the electriccharges at the both side regions shown in FIGS. 18A and 18C through thesubstrate 50.

By the way, parts of electric charges generated at the side gateelectrodes 21 a, 21 c may flow into the potential well 27 correspondingto the center gate electrode 21 b as noise components during a timeperiod that the electric charges are generated by the photodiode 21. Inaddition, since the signal charges are transferred every time that thesignal charges corresponding to one intensity (A0, A1, A2, A3) ofreceived light can be detected, the electric charges generated by thephotodiode 21 may be mixed as the noise components during a time periodof transferring the signal charges. However, these noise components areaveraged by integration, and substantially removed by subtractionperformed to determine the phase difference “ψ”. Therefore, theinfluence of these noise components becomes small. As a result, it ispossible to accurately determine the phase difference “ψ” even whenusing the frame transfer CCD.

In the above case, the three gate electrodes 21 a˜21 c are formed everyphotodiode 21. However, the number of gate electrodes corresponding toone photodiode is not limited to this embodiment. Other configurationand performance are substantially the same as the fourth embodiment.

<Seventh Embodiment>

This embodiment explains about a case of using a frame transfer CCDhaving a lateral-overflow drain, which is available in the market, asthe image sensor 1 of FIG. 1.

As shown in FIG. 19, the image sensor 1 of this embodiment is a2-dimensional image sensor having a matrix (4×4) arrangement ofphotodiodes 21.

An overflow drain 61 of n-type semiconductor is formed for each ofcolumns of the matrix arrangement of the photodiodes 21. In thisembodiment, the image sensor 1 has four overflow drains 61. The overflowdrains 61 are connected at their upper ends to each other by an overflowelectrode 24 that is an aluminum electrode. The image pickup portion“D1”, the storage portion “D2” and the horizontal transfer portion 23have the same functions to them of the image sensor 1 of the sixthembodiment.

Referring to FIG. 20, the image sensor 1 of this embodiment is morespecifically explained. That is, a p-type semiconductor substrate isused as the substrate 60. A p-type semiconductor layer 62 is formed on ageneral surface of the substrate 60. In addition, an n-well 63 of n-typesemiconductor is formed in the p-type semiconductor layer 62. Therefore,the photodiode 21 is composed of the p-type semiconductor layer 62 andthe n-well 63 provide. A p⁺-well 64 of p⁺-type semiconductor is formedadjacent to the n-well 63 in the p-type semiconductor layer 62. Theoverflow drain 61 of n-type semiconductor is formed in a top surface ofthe p⁺-well 64. Thus, the image sensor 1 of this embodiment hasbasically same structure as the image sensor of the sixth embodimentexcept for using the substrate 60 having a different conductivity typeand forming the overflow drain 61. As compared with the sixthembodiment, there is an advantage that the image sensor of thisembodiment has a higher sensitivity to infrared.

As understood from comparison between FIG. 21 showing a change inelectron potential along the dotted line “L4” of FIG. 20 and FIG. 16,the present embodiment is different from the sixth embodiment in thatthe electric charges generated by the photodiode 21 are discardedthrough the overflow drain 61 in place of the substrate 50 of the sixthembodiment. Amounts of electric charges stored in a potential well,which is formed in the n-well 63 by applying voltages to gate electrodes21 a˜21 c, are determined by the depth of the potential well, which canbe changed by controlling magnitudes of the voltages applied to the gateelectrodes 21 a˜21 c. That is, when the magnitude of the voltage appliedto the gate electrode 21 b is higher than the magnitudes of the voltagesapplied to the gate electrodes 21 a, 21 c, the potential wellcorresponding to the center gate electrode 21 b has a maximum depth.

When a suitable voltage is applied to the overflow electrode 24 to lowerthe potential barrier “B3”, electric charges (electrons “e”) are storedin the potential well corresponding to the center gate electrode 21 b,as shown in FIG. 22B, and electric charges generated at the regionscorresponding to the side gate electrodes 21 a, 21 c are discarded bythe overflow drain 61, as shown in FIGS. 22A and 22C. Otherconfiguration and performance are substantially the same as the sixthembodiment.

<Eighth Embodiment>

As explained in the sixth and seventh embodiments, when using the frametransfer CCD as the image sensor 1, the number of gate electrodes formedevery photodiode 21 is not limited to three. In this embodiment, fourgate electrodes are formed every photodiode 21.

In FIGS. 23A and 23B, the numerals 1˜4 respectively designate first,second, third and fourth gate electrodes. One cycle of the numerals 1˜4corresponds to one photodiode 21. FIG. 23A shows a charge storing periodof storing electric charges generated by the photodiode 21, and FIG. 23Bshows a charge discarding period of discarding dispensable charges. Athreshold value “Th1” designates a potential of overflow drain.

As shown in FIG. 23A, in the charge storing period, a potential barrieris formed between adjacent photodiodes 21 by not applying a voltage tothe first gate electrode (1) in order to prevent that that electriccharges generated by the photodiodes 21 are mixed to each other. Inaddition, when magnitudes of voltages applied to the second, third andthe fourth gate electrodes (2)˜(4) are changed in a stepwise manner, itis possible to obtain a step-like potential well having differentdepths. Each of regions of the potential well corresponding to the thirdand fourth gate electrodes (3), (4) has an electron potential higherthan the threshold value “Th1”. The potential well has a maximum depth(the lowest electron potential) at a region corresponding to the secondgate electrode (2). Since this electron potential is lower than thethreshold value “Th1”, electric charges (electrons “e”) generated by thephotodiode 21 are mainly stored in the region corresponding to thesecond gate electrode (2).

On the other hand, as shown in FIG. 23B, in the charge discardingperiod, electron potentials of the regions corresponding to the thirdand fourth gate electrodes (3), (4) are increased to prevent a leakageof electric charges stored in the region corresponding to the secondgate electrode (2) that has the lowest electron potential within thecharge storing period. Thereby, electric charges generated at theregions corresponding to the first, third and the fourth gate electrodes(1), (3), (4) in the charge storing period flow into the overflow drainand the region corresponding to the second gate electrode (2). In otherwords, in the charge storing period shown in FIG. 23B, the voltagesapplied to the gate electrodes (1) to (4) are controlled to formpotential barriers for electrically isolating the region correspondingto the second gate electrode (2) having the lowest electron potentialfrom the circumstances. Therefore, it is possible to change a ratio ofthe dispensable charges to be discarded relative to the electric chargesgenerated by the photodiode 21 by adequately controlling a ratio of thecharge storing period to the charge discarding period. This means thatthe sensitivity is adjustable. Other configuration and performance aresubstantially the same as the sixth or seventh embodiment.

<Ninth Embodiment>

In this embodiment, six gate electrodes are formed every photodiode 21.In FIGS. 24A and 24B, the numerals 1˜6 respectively designate first,second, third, fourth, fifth and sixth gate electrodes. One cycle of thenumerals 1˜6 corresponds to one photodiode 21. FIG. 24A shows a chargestoring period of storing electric charges generated by the photodiode21, and FIG. 24B shows a charge discarding period of discardingdispensable charges. A threshold value “Th2” designates a potential ofoverflow drain.

As shown in FIG. 24A, in the charge storing period, a potential barrieris formed between adjacent photodiodes 21 by not applying a voltage tothe first gate electrode (1) in order to prevent that electric chargesgenerated by the photodiodes 21 are mixed to each other. The potentialwell has a maximum depth at a region corresponding to the fourth gateelectrode (4). Electron potentials of regions corresponding to thesecond, third, fifth and the sixth gate electrodes (2), (3), (5), (6)are changed in a stepwise manner so as to be higher than the thresholdvalue “Th2”. Since the region of the potential well corresponding to thefourth gate electrode (4) has the maximum depth, which is lower than thethreshold value “Th2”, electric charges (electrons “e”) generated by thephotodiode 21 are mainly stored in this region.

FIG. 25A is an explanatory view of 3-dimensionally illustrating theelectron potentials of the regions corresponding to the gate electrodes(1) to (6) in the charge storing period. “V1” to “V6” shown in thisfigure respectively correspond to vertical transfer voltages “V1” to“V6” shown in FIG. 19, and “LOD” corresponds to overflow drains 61.Electrons lying in regions corresponding to the gate electrodes (2) and(6), to which the voltages “V2” and “V6” are being applied, migratetoward regions corresponding to the gate electrodes (3) and (5), towhich the voltages “V3” and “V5” are being applied. In addition, theelectrons lying in the regions corresponding to the gate electrodes (3)and (5) migrate toward a region corresponding to the gate electrode (4),to which the voltage “V4” is being applied. As a result, the electronsare stored in the region of the potential well corresponding to the gateelectrode (4).

On the other hand, as shown in FIG. 24B, in the charge discardingperiod, electron potentials of the regions corresponding to the second,third, fifth and sixth gate electrodes (2), (3), (5), (6) are increasedto prevent a leakage of electric charges stored in the regioncorresponding to the fourth gate electrode (4) that has the lowestelectron potential within the charge storing period. Thereby, electriccharges generated at the regions corresponding to the first, second,third, fifth and sixth gate electrodes (1), (2), (3), (5), (6) in thecharge storing period flow into the overflow drain and the regioncorresponding to the fourth gate electrode (4).

FIG. 25B is an explanatory view of 3-dimensionally illustrating theelectron potentials of the regions corresponding to the gate electrodes(1) to (6) in the charge discarding period. When the voltages “V3” and“V5” are reduced so as to be substantially equal to the voltage “V”, theregion corresponding to the gate electrode (4), to which the voltage“V4” is being applied, is electrically isolated from the circumstances,so that an inflow of electrons to the region is inhibited. In addition,by setting the electron potential of the overflow drain (LOD) electrodeto be higher than the gate electrode (4), to which the voltage “V4” isbeing applied, and lower than the gate electrodes (2), (6), to which thevoltages “V2”, “V6” are being applied, it is possible to discard theelectrons generated at the regions corresponding to the gate electrodes(2), (6) to the overflow drain electrode without discarding theelectrons stored in the region corresponding to the gate electrode (4).

Therefore, as in the case of the eighth embodiment, it is possible tochange a ratio of the dispensable charges to be discarded relative tothe electric charges generated by the photodiode 21 by appropriatelycontrolling a ratio of the charge storing period to the chargediscarding period. This means that the sensitivity is adjustable. Otherconfiguration and performance are substantially the same as the sixth orseventh embodiment.

<Tenth Embodiment>

As described above, when using the frame transfer CCD as the imagesensor 5, electric charges generated by the photodiode 21 in timeperiods other than the time periods of extracting the signal chargescorresponding to the intensities (A0˜A3) of received light may be mixedas the noise components into the signal charges. The noise componentsare substantially constant, and averaged by storing the electric chargesthe time periods of extracting the signal charges corresponding to theintensities (A0˜A3) of received light. Therefore, it is possible toremove the noise components to some extent and determine the phasedifference “ψ”.

However, the S/N ratio reduces due to the noise components. For example,when a larger dynamic range is needed at regions for storing ortransferring the electric charges, the cost/performance of the distancemeasuring apparatus deteriorates. In the present embodiment, as shown inFIGS. 26A and 26B, light-shielding films 65 are formed on the region ofstoring the signal charges and the region not relating to the generationof electric charges of the photodiodes 21.

In FIGS. 26A and 26B, as in the case of the ninth embodiment, six gateelectrodes (1) to (6) are formed every photodiode 21. Specifically, thelight-shielding film 65 is formed at the regions corresponding to thegate electrodes (1) and (4), so that electric charges (electrons “e”)are generated only at the regions corresponding to the gate electrodes(2), (3), (5) and (6) of the respective photodiode 21. As a result, thegate electrode (4) does not substantially contribute the generation ofelectric charges. In other words, the noise components do not occur atthe gate electrode (4). Therefore, as compared with the case of notforming the light-shielding film 65, it is possible to improve the S/Nratio. Alternatively, the light-shielding film 65 may be formed at theregions corresponding to the gate electrodes (1), (3), (4) and (5), sothat electric charges (electrons “e”) are generated only at the regionscorresponding to the gate electrodes (2) and (6) of the photodiode 21.Other configuration and performance are substantially the same as theninth embodiment.

In the fourth or tenth embodiment, the timing of applying the voltagesto the first and second electrodes were controlled by the methodexplained in the first embodiment. Alternatively, the method explainedin the second or third embodiment may be used.

In the above-explained embodiments, the electric charges are outputevery time that the signal charges corresponding to one intensity (A0,A1, A2, A3) of received light are detected. According to an image sensor1 explained below, it is possible to simultaneously detect the signalcharges corresponding to at least two intensities (A0, A1, A2, A3) ofreceived light.

<Eleventh Embodiment>

In this embodiment, a modification of the frame transfer CCD having theoverflow-drain of FIG. 19 is used as the image sensor 1. That is, asshown in FIG. 27, overflow drains (61 a, 61 b) are alternately disposedsuch that each of the overflow drains is formed every photodiode 21.Therefore, the electric charges generated by each of the photodiodes 21can be separately discarded.

A ratio of the signal charges supplied into the potential well of thecharge storage portion relative to the electric charges generated by thephotodiode 21 can be changed by applying the voltage to each of theoverflow drains 61 a, 61 b in synchronization with the period of themodulation signal. In this embodiment, the timing of applying thevoltage to the overflow drain 61 a is different by 180 degrees from thetiming of applying the voltage to an adjacent overflow drain 61 b in adirection of transferring the signal charges. Thus, by applying the twovoltages having different phases (φ1, φ2) to the overflow drains 61 a,61 b, the signal charges corresponding to these two phases of themodulation signal, which are different from each other by 180 degrees,can be respectively stored in the potential wells formed in thecorresponding photodiodes 21.

That is, the signal charges corresponding to two of four intensities(A0˜A3) of received light required to determine the phase difference “ψ”can be simultaneously extracted. For example, the signal chargescorresponding to the intensities (A0 and A3) of received light aresimultaneously extracted, and then the signal charges corresponding tothe intensities (A1 and A2) of received light are simultaneouslyextracted.

In this embodiment, since the signal charges are mixed with extraneouselectric charges with an intended purpose, they become noise components.However, amounts of the extraneous electric charges are much smallerthan the signal charges, and the extraneous electric charges are mixedat a substantially constant ratio with the signal charges. Therefore,the noise components have a minimal influence on determining the phasedifference “ψ”. Other configuration and performance are substantiallythe same as the seventh embodiment.

<Twelfth Embodiment>

In this embodiment, a modification of the interline transfer CCD havingthe lateral overflow-drain is used as the image sensor 5 in place of themodification of the frame transfer CCD. That is, as shown in FIG. 28,overflow drains (41 a, 41 b) are alternately disposed adjacent to thephotodiodes 21 aligned in the vertical direction such that each of theoverflow drains is provided every photodiode 21. In the verticaltransfer portion 22, three gate electrodes 22 a˜22 c are provided everyphotodiode 21.

In this image sensor 1, voltages are applied at different phases of themodulation signal to the adjacent overflow drains 41 a, 41 b in thevertical direction. In addition, as in the case of the eleventhembodiment, the gate electrodes 22 a˜22 c are driven by 6 phase controlvoltages. As a result, it is possible to simultaneously obtain thesignal charges corresponding to two of the four intensities (A0˜A3) ofreceived light. Other configuration and performance are substantiallythe same as the eleventh embodiment.

<Thirteen Embodiment>

In the eleventh and twelfth embodiments, three gate electrodes areprovided every overflow drain. However, the number of gate electrodesprovided every overflow drain may be four or more. In addition, whenvoltages are applied to four different overflow drains at four timings,which are different from each other by 90 degrees with respect to thephase of the modulation signal, it is possible to simultaneously obtainthe signal charges corresponding to four intensities (A0˜A3) of receivedlight. Moreover, the timing of applying each of voltages may be aspecific phase synchronized with the period of the modulation signal. Aninterval between the timings of applying the voltages can be optionallydetermined.

In the above embodiments, the interline transfer CCD or the frametransfer CCD were used as the image sensor 1. In addition, a frameinterline transfer CCD may be used, which is obtained by replacing theimage pickup portion “D1” of the frame transfer CCD of FIG. 14 withphotodiodes 21 and vertical transfer portions 22 of the interlinetransfer CCD, as shown in FIG. 29. In this case, there is an advantageof preventing the occurrence of smear, as compared with the frametransfer CCD.

In the above-explained embodiments, an image sensor having onedimensional array of photodiodes may be used in place of the imagesensor 1. In addition, only one photoelectric converter 11 may be usedin the first embodiment. The analyzer used in the above embodimentsprovides the distance information. However, as the informationconcerning the intended space, only the phase difference “ψ” may bedetermined. Alternatively, the analyzer may determine another spatialinformation concerning the intended space according to the intensity ofreceived light.

INDUSTRIAL APPLICABILITY

As described above, according to the spatial information detectingapparatus of the present invention, by controlling the voltage appliedto an electrode of the charge discarding portion at the timingsynchronized with the period of a modulation signal, it is possible tosurely remove residual electric charges, which are dispensable chargesnot used as signal charges, of the electric charge generated by thephotoelectric converter before they are transferred to charge storageportion, and as a result remarkably improve S/N ratio.

In the case of using a spatial information detecting apparatus with aconventional CCD image sensor having an overflow drain electrode, it ispossible to adequately control the sensitivity of the CCD image sensorby removing dispensable charges from electric charges generated byphotoelectric converters of the CCD image sensor according to a controlvoltage applied to the overflow electrode in synchronization with theperiod of the modulation signal, and storing the balance of the electriccharges as the signal charges in a charge storage area of the CCD imagesensor.

The spatial information detecting apparatus of the present invention iswidely available to any devices required to determine a phase differencebetween the intensity-modulated light and the received light, andparticularly suitable to the distance measuring apparatus.

1. A spatial information detecting apparatus using intensity-modulatedlight comprising: at least one photoelectric converter for receiving alight provided from a space, into which a light intensity-modulated by apredetermined modulation signal is being irradiated, and generatingamounts of electric charges corresponding to an intensity of receivedlight; charge discarding means having a first electrode for removingdispensable charges from the electric charges generated by saidphotoelectric converter according to a voltage applied to said firstelectrode; charge storage means for storing signal charges from theelectric charges generated by said photoelectric converter; a controlcircuit for controlling the voltage applied to said first electrode at atiming synchronized with a period of said modulation signal to change aratio of the signal charges stored in said charge storage means to theelectric charges generated by said photoelectric converter; a chargeejector for outputting the signal charges from said charge storagemeans; and an analyzer for determining spatial information from anoutput of said charge ejector.
 2. The spatial information detectingapparatus as set forth in claim 1, wherein said charge storage means hasa second electrode, and said control circuit controls a voltage appliedto said second electrode constant to transfer required amounts of theelectric charges generated by said photoelectric converter to saidcharge storage means.
 3. The spatial information detecting apparatus asset forth in claim 2, wherein said control circuit controls the voltagesapplied to said first electrode and said second electrode so as toalternately switch between a stage of transferring the electric chargesgenerated by said photoelectric converter to said charge storage meansand a stage of transferring the electric charges generated by saidphotoelectric converter to said charge discarding means.
 4. A spatialinformation detecting apparatus using intensity-modulated lightcomprising: at least one photoelectric converter for receiving a lightprovided from a space, into which a light intensity-modulated by apredetermined modulation signal is being irradiated, and generatingamounts of electric charges corresponding to an intensity of receivedlight; charge discarding means having a first electrode for removingdispensable charges from the electric charges generated by saidphotoelectric converter according to a voltage applied to said firstelectrode; charge storage means having a second electrode for storingsignal charges from the electric charges generated by said photoelectricconverter according to a voltage applied to said second electrode; acontrol circuit for controlling the voltage applied to said secondelectrode at a timing synchronized with a period of said modulationsignal, while applying a constant voltage to said first electrode, tochange a ratio of the signal charges stored in said charge storage meansto the electric charges generated by said photoelectric converter; acharge ejector for outputting the signal charges from said chargestorage means; and an analyzer for determining spatial information froman output of said charge ejector.
 5. The spatial information detectingapparatus as set forth in claim 1, wherein said at least onephotoelectric converter is a plurality of photoelectric converters, andthe spatial information detecting apparatus includes a CCD image sensorhaving said photoelectric converters, said charge storage means and saidcharge ejector, and wherein said CCD image sensor has an overflow drainas said charge discarding means.
 6. The spatial information detectingapparatus as set forth in claim 1, wherein said at least onephotoelectric converter is a plurality of photoelectric converters, aset of photoelectric converters is selected from said photoelectricconverters to define one pixel, said control circuit allows said chargestorage means to store the signal charges from the electric chargesgenerated by each of said photoelectric converters of the set at atiming of each of different phases in synchronization with the period ofsaid modulation signal, and wherein said charge ejector simultaneouslyoutputs the signal charges stored with respect to the different phases.7. The spatial information detecting apparatus as set forth in claim 2,wherein said charge storage means has a light shielding film on saidsecond electrode formed in the vicinity of a region of storing thesignal charges.
 8. The spatial information detecting apparatus as setforth in claim 1, wherein said analyzer determines a phase differencebetween the light irradiated into the space and the light received bysaid photoelectric converter from the signal charges stored with respectto different phases of said modulation signal.
 9. The spatialinformation detecting apparatus as set forth in claim 8, wherein saidanalyzer converts said phase difference into distance information. 10.The spatial information detecting apparatus as set forth in claim 1,wherein said analyzer determines distance information from the signalcharges stored with respect to different phases in a period of saidmodulation signal, and wherein the spatial information detectingapparatus further comprises a phase switch for changing the phase ofsaid modulation signal, at which the voltage is applied to said firstelectrode, every time that storing the signal charges in said chargestorage means at the phase is finished.
 11. A spatial informationdetecting method using a CCD image sensor having an overflow drainelectrode comprising the steps of: allowing said CCD image sensor toreceive a light provided from a space, into which a lightintensity-modulated by a predetermined modulation signal is beingirradiated; storing signal charges by repeating a charge extractionoperation plural times with respect to each of different phases in aperiod of said modulation signal; and determining spatial informationfrom the signal charges stored with respect to the different phases ofsaid modulation signal, wherein said charge extraction operationincludes the steps of removing dispensable charges from electric chargesgenerated by photoelectric converters of said CCD image sensor accordingto a control voltage applied to said overflow electrode insynchronization with the period of said modulation signal, and storingthe balance of the electric charges as the signal charges in a chargestorage area of said CCD image sensor.
 12. The spatial informationdetecting method as set forth in claim 11, wherein said CCD image sensoris an interline transfer CCD image sensor.
 13. The spatial informationdetecting method as set forth in claim 11, wherein said CCD image sensoris a frame transfer CCD image sensor.
 14. The spatial informationdetecting method as set forth in claim 13, wherein said CCD image sensorhas at least three photoelectric converters, and said charge extractionoperation includes the step of applying the control voltage to saidoverflow drain electrode in synchronization with the period of saidmodulation signal such that the electric charges generated by apredetermined one(s) of said at least three photoelectric converters arestored as the signal charges in said charge storage area, and theelectric charges generated by the remaining photoelectric converter(s)are discarded as the dispensable charges.
 15. The spatial informationdetecting method as set forth in claim 14, wherein the control voltageis applied to said overflow drain electrode to generate a potentialbarrier for electrically isolating the predetermined photoelectricconverter(s) from the remaining photoelectric converter(s).
 16. A lightreceiving element with controllable sensitivity comprising: at least onephotoelectric converter for receiving a light provided from a space,into which a light intensity-modulated by a predetermined modulationsignal is being irradiated, and generating amounts of electric chargescorresponding to an intensity of received light; charge discarding meanshaving an electrode for removing dispensable charges from the electriccharges generated by said photoelectric converter according to a voltageapplied to said electrode; charge storage means for storing signalcharges from the electric charges generated by said photoelectricconverter; a sensitivity controller for controlling the voltage appliedto said electrode at a timing synchronized with a period of saidmodulation signal to change a ratio of the signal charges stored in saidcharge storage means to the electric charges generated by saidphotoelectric converter; and; a charge ejector for outputting the signalcharges from said charge storage means.